Dual-energy scanning-based detection of ionizing radiation

ABSTRACT

A dual-energy scanning-based radiation detecting apparatus comprises a line detector; a device for scanning said line detector across an object while said line detector is exposed to an ionizing radiation beam, which has impinged on said object, to thereby record a plurality of line images of said object; a filter device arranged in the path of said ionizing radiation beam upstream of said object to filtrate said ionizing radiation beam, the filter device being capable of operating in two operation modes having different filter characteristics; and a control device for altering operation mode of the filter device subsequent to at least every second of said line images being recorded.

FIELD OF THE INVENTION

[0001] The invention relates generally to apparatuses and methods fordual-energy scanning-based detection of radiation.

BACKGROUND OF THE INVENTION AND RELATED ART

[0002] Various line detectors for detecting ionizing radiation are knownin the art. While such detectors provide for instantaneousone-dimensional imaging, two-dimensional imaging can only be performedby means of scanning the line detector, and optionally the radiationsource, in a direction traverse to the one-dimensional detector array.Such scanning-based detection may be time consuming. Movement of theobject being examined may occur during scanning, which would severelyreduce the image quality obtained.

[0003] There are also known dual-energy detectors in the art, i.e.detectors, with which two images are produced using radiation ofdifferent energy and combined into a single image to enhance differentelements in the image. Generally attenuation is a function of x-rayenergy according to the two attenuation mechanisms photoelectricabsorption and Compton scattering. These two mechanisms differ amongmaterials of different atomic numbers. For this reason, measurements attwo energies can be used to distinguish between different elements.

[0004] Dual-energy x-ray techniques can be used to identify bone tissueseparately from soft tissue in medical imaging, for example, or toidentify hazardous materials, for example, in baggage scanning.

SUMMARY OF THE INVENTION

[0005] However, when dual-energy imaging measurements are performedusing a line detector, the object to be imaged has to be scannedtwice—one time using radiation having a first radiation spectrum andthen another time using radiation having a second radiation spectrum. Toretrieve material-specific information the two images are compared ateach position, i.e. on a pixel-based basis. The time lapsed between twodetections at each position corresponds to the total scanning time forobtaining one two-dimensional image. This period of time may be large,e.g. several seconds, during which the object might have moved or havebeen moved. Particularly, when imaging living organisms, or portionsthereof, it is expected that a considerable movement might have occurredon the time scale in question. Such movement renders the dual-energycomparison useless, or at least of significantly reduced quality.

[0006] Another option is to use two different radiation sourcesproducing radiation of different energy. However, an additionalradiation source adds to the cost for the equipment, and furthermore thesources have to be placed a certain distance from each other due totheir size, and thus it is difficult to image the same point in theobject simultaneously (or nearly simultaneously).

[0007] Still another option is to use a single radiation source, butalter its operation voltage between two different settings, at whichradiation of different energy is produced. However, this adds a furthertime delay since the operation voltage of the radiation source cannot bealtered instantaneously, and thus movement of the object may occurbetween the recordings of succeeding images.

[0008] A main object of the invention is therefore to provide adual-energy scanning-based ionizing radiation detecting apparatus andmethod, which overcome or at least reduce the problem described above.

[0009] In this respect there is a particular object to provide such anapparatus and such a method, which are uncomplicated and can producedual-energy high-quality two-dimensional images with excellent,signal-to-noise ratio, dynamic range, and image contrast.

[0010] A further object of the invention is to provide such an apparatusand such a method, which enable a fast scanning across the object to beexamined.

[0011] A yet further object of the invention is to provide such anapparatus and such a method, which are reliable, accurate, andinexpensive.

[0012] These objects, among others, are attained by apparatuses andmethods as claimed in the appended claims.

[0013] The inventors have found that by providing a filter devicearranged in the path of a ionizing radiation beam upstream of an object,e.g. patient, to be examined in a dual-energy scanning-based measurementto filtrate the ionizing radiation beam, where the filter device iscapable of operating in two or more operation modes having differentfilter characteristics, and wherein a control device alters operationmode of the filter device subsequent to at least every second of anumber of line images being recorded by the scanning-based ionizingradiation detecting apparatus, a dual- (or multiple-) energyscanning-based imaging technique is obtained, where any movement of theexamination object during scanning will affect the dual-energy imagesidentically or at least similarly.

[0014] The inventors have also found that by using a scanning-basedradiation detector apparatus comprising a plurality of stacked linedetectors, each exposed to an ionizing radiation beam, a filter devicemay be arranged in the path of the radiation beams upstream of an objectto be examined to filtrate the radiation beams, where the filter devicecomprises an array of filter sections aligned with the radiation beamsso that each of the radiation beams will have been filtered by arespective one of the filter sections when impinging on the object,where every second filter section in the array has a first filtercharacteristic and each other filter section in the array has a secondfilter characteristic, and scanning may be performed with the array offilter sections kept aligned with the radiation beams across a distancecorresponding to two times the distance between two adjacent linedetectors of the stacked line detectors. Hereby, an high-qualitydual-energy scanning-based imaging measurement is obtained, wherecorresponding pixels in the two images are recorded close in time tominimize problems in the dual-energy evaluation due to object movements.

[0015] The one-dimensional detector unit is preferably, but notexclusively, a gaseous based parallel plate detector unit. Otherdetector units that may be used include diode arrays, scintillator basedarrays, CCD arrays, TFT- and CMOS-based detectors, liquid detectors, andsolid-state detectors, e.g. one-dimensional PIN-diode arrays withedge-on, near edge-on or perpendicular incidence of X-rays.

[0016] Further characteristics of the invention, and advantages thereof,will be evident from the detailed description of preferred embodimentsof the present invention given hereinafter and the accompanying FIGS.1-9, which are given by way of illustration only, and thus are notlimitative of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 illustrates schematically, in a side view, an apparatus fordual-energy scanning-based X-ray imaging according to a preferredembodiment of the present invention.

[0018]FIG. 2 is a schematic enlarged cross-sectional view of some of thecomponents of the apparatus of FIG. 1 taken along the line A-A.

[0019]FIG. 3 is a schematic enlarged cross-sectional view of maincomponents of an apparatus for dual-energy scanning-based X-ray imagingaccording to still a preferred embodiment of the invention.

[0020]FIG. 4 is a schematic top plan view of a detector arrangement tobe used in an apparatus for dual-energy scanning-based X-ray imagingaccording to yet a preferred embodiment.

[0021]FIG. 5 is a schematic top plan view of a collimator arrangement tobe used with the detector arrangement of FIG. 4.

[0022]FIG. 6 is a schematic top plan view of a filter arrangement to beused with the detector arrangement of FIG. 4.

[0023]FIG. 7 is a schematic enlarged cross-sectional view of maincomponents of an apparatus for dual-energy scanning-based X-ray imagingaccording to still a preferred embodiment of the invention.

[0024]FIG. 8 is a schematic top plan view of a filter arrangement to beused in an apparatus for dual-energy scanning-based X-ray imagingaccording to yet a preferred embodiment.

[0025]FIG. 9 is a schematic top plan view of a filter arrangement to beused in an apparatus for dual-energy scanning-based X-ray imagingaccording to still a preferred embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] From top to bottom the apparatus in FIG. 1 comprises an X-raysource 11, a common filter 12, a dual- (or multiple-) energy filterdevice 14, a fan beam collimator 13, an object table or holder 15, and aone-dimensional detector unit 16.

[0027] The X-ray source 11 is a conventional X-ray tube having acathode, which emits electrons, and an anode emitting X-rays in responseto being struck by the electrons.

[0028] The common filter 12 is preferably located just beneath the X-raytube 11, which typically includes thin metallic foils acting as filtersto absorb the lowest (and sometimes also the highest) energy photons,which do not contribute significantly to the image quality. This filteris optional and can be part of the dual-energy filter device 14,described below.

[0029] The dual- (or multiple-) energy filter device 14 is preferablylocated just above the collimator 13. The filter device 14 has avariable spectral transmission characteristic to be discussed in detailfurther below.

[0030] The fan beam collimator 13, which is optional, may be a thin foilof e.g. tungsten with a narrow radiation transparent slit etched away.The slit is aligned with a corresponding line-shaped sensitive area orentrance slit of the detector unit 16 so that X-rays passing through theslit of the fan beam collimator 13 will reach the sensitive area of thedetector unit 16.

[0031] Yet optionally, a further collimator is arranged in front of thedetector (i.e. downstream of an object to be imaged).

[0032] The detector unit 16 is illustrated more in detail in FIG. 2 andis oriented so that a planar or fan-shaped X-ray beam 24 can entersideways between essentially planar cathode and anode arrangements. Eachof the electrode arrangements includes an electrically conductingelectrode layer 25, 27 supported by a respective dielectric substrate26, 28, wherein the arrangements are oriented with the conductivecathode 25 and anode 27 layers facing each other. A radiationtransparent window 30 is provided at the front of the detector unit toform an entrance for the fan-shaped beam 24 to the detector unit 16.

[0033] Preferably, the dielectric substrates 26, 28 and the window 30define together with sidewalls 29 a gas-tight confinement capable ofbeing filled with a gas or gas mixture, which is ionized by the incidentradiation. Alternatively, the electrode arrangements are arranged withinan external gas-tight casing (not illustrated).

[0034] A voltage is applied across the electrode arrangements to driftthe electrons freed as a result of ionization towards the anodearrangement.

[0035] The detector unit 16 comprises further a readout arrangementincluding a one-dimensional array of individual readout elements forrecording a one-dimensional image of the fan-shaped beam 24. Typically,the readout arrangement is integrated with the anode arrangement 27, 28.The detector unit 16 may also comprise capabilities for electronavalanche amplification in order to record very low flux of X-rays, ordetect each single X-ray with high efficiency.

[0036] The X-ray tube 11, the common filter 12, the dual-energy filterdevice 14, the fan beam collimator 13 and the detector unit 16 areattached to a common E-arm 17, which in turn is rotatably attached to avertical stand 18 by means of a spindle 19 approximately at the heightof the X-ray tube 11. In this manner, the X-ray tube 11, the commonfilter 12, the filter device 14, the fan beam collimator 13 and thedetector unit 16 can be moved in a common pivoting movement relative toan object to be examined arranged on the object table 15 to scan theobject and produce a two-dimensional image thereof. The pivotingmovement is schematically indicated by arrow 23. The object table 15 isfirmly attached to a support 20, which in turn is firmly attached to thevertical stand 18. For this purpose the E-arm 17 is provided with arecess or similar in the E-arm 17 (illustrated by the dashed lines).During scanning, the object is kept still.

[0037] It shall be appreciated that the detector apparatus of FIG. 1 maybe modified and arranged for linear movement of the X-ray tube 11, thecommon filter 12, the filter device 14, the fan beam collimator 13 andthe detector unit 16 with respect to the object being examined. Suchlinear scanning movement is schematically indicated by arrow 23 a inFIG. 2. Yet alternatively, the common filter 12, the filter device 14,the fan beam collimator 13 and the detector unit 16 may be rotated inthe horizontal plane with respect to the object being examined as beingschematically indicated by arrow 23 b in FIG. 2. Such rotational-basedscanning is disclosed in U.S. Pat. No. 6,067,342 (Gordon) and U.S. Pat.No. 5,025,376 (Bova et al.), the contents of which being herebyincorporated by reference.

[0038] It shall further be appreciated that the apparatus of FIG. 1 maybe modified such that the object is moved during scanning, while theX-ray tube 11, the common filter 12, the filter device 14, the fan beamcollimator 13 and the detector unit 16 are kept at rest.

[0039] Furthermore, the detector apparatus comprises a microprocessor orcomputer 21 provided with suitable software for controlling theapparatus and readout and post-processing of the signals from the linedetector unit 16 and a power supply 22 for supplying the detector unitand the microprocessor or computer 21 with power and for driving a stepmotor or similar housed in the vertical stand 18 for driving the spindle19 and thus the E-arm 17.

[0040] In operation, X-rays are emitted from the X-ray tube 11 and passthrough the common filter 12, and the filter device 14. Only x-rayspassing through the slit of the fan beam collimator 13 traverse theobject. In the object, the X-ray photons can be transmitted, absorbed orscattered. The X-rays that are transmitted leave the object and enterinto the detector unit 16 and are detected. From the detection aone-dimensional image of the object is formed.

[0041] It shall be appreciated by the person skilled in the art thatinstead of detecting X-rays that are transmitted, the detector apparatusmay be arranged to detect X-rays that are scattered in somepredetermined angle.

[0042] During scanning, the E-arm 17, holding the X-ray source 11, thecommon filter 12, the filter device 14, the fan beam collimator 13 andthe detector unit 16, are moved in a pivoting movement such that thedetector unit scans across the object in a direction, which isessentially parallel with the object table 15. At regular movementintervals, i.e. scanning distances ss, the detected signals are read outand stored in a memory of the microprocessor 21. When the scanning isstopped, a number of one-dimensional images of the object have beenobtained, which are grouped together by the microprocessor 21 to createa two-dimensional image of the object. According to an alternativescanning technique the E-arm 17 is moved relative the object stepwise,and the one-dimensional detector unit 16 is detecting, while being stillbetween the stepwise movements.

[0043] According to the present invention a dual-energy scanning isperformed, which generally requires a specifically adapted filterdevice. To this end the filter device 14 is capable of operating in twooperation modes having different filter characteristics.

[0044] One filter transmits preferably a low energy X-ray spectrum withaverage energy of typically 40 keV. This filter may be made of one orseveral thin foils of different elements. The filter elements are chosensuch that the high-energy part of the incoming X-ray spectrum isabsorbed in the filter. This can be achieved by using filter elementsthat have highly absorbing K-shell energies above the desiredtransmitted X-ray spectrum, e.g. cesium, barium, some of the lanthanidesand/or heavier elements such as tungsten, gold or lead (with atomicnumber typically higher than 54).

[0045] The other filter transmits preferably a high-energy spectrum withaverage energy of typically 70 keV or higher. This filter may be made ofone or several thin foils of different elements. The filter elements arechosen such that the low energy part of the incoming X-ray spectrum isabsorbed in the filter. This can be achieved by using filter elementsthat have highly absorbing K-shell energies below the desiredtransmitted X-ray spectrum, e.g. cupper, molybdenum, silver etc. (withatomic number typically less than 64).

[0046] A control device is provided for altering operation mode of thefilter device 14 subsequent to at least every second of the line imagesbeing recorded, e.g. every second line image is recorded using filteredradiation having a first radiation spectrum and the other line imagesare recorded using filtered radiation having a second radiationspectrum, or a repeated series of two line images being recorded usingfiltered radiation having the first radiation spectrum followed by twoline images being recorded using filtered radiation having the secondradiation spectrum is performed.

[0047] Typically, the filter device 14 is provided with two filtersections 14 a, 14 b as shown in FIG. 2, which sections have separatefilter functions and which are capable of being arranged—one at atime—in the path of the radiation beam 24. A moving mechanism, e.g.housed in the E-arm 17, may be implemented to move the filter device 14in directions of the bi-directional arrow 25 under control of thecomputer 21 to alternately arrange the two filter sections in the pathof the radiation beam. Note that this movement will be superimposed onthe scanning movement.

[0048] If the scanning is performed with a continuous scanning movement,preferably every second line image is recorded using filtered radiationhaving a first radiation spectrum and every second line image isrecorded using filtered radiation having a second radiation spectrum.Provided that the scanning step is short, the misalignment, i.e. thepixel-to-pixel correspondence, in the dual-energy image evaluation issmall, and provided that the detection time is short, movement of theobject between the completion of the recordings of two subsequent lineimages is not occurring to any appreciable extent.

[0049] For instance, provided a scanning step of 50 microns and a lineimage exposure time of 1 ms, the misalignment in the pixel-to-pixelcomparison will be about 50 microns (i.e. the two two-dimensional imagesare displaced on average 50 microns relative each other, whereas thetwo-line detection time will be 2 ms (if the time of detectorelectronics readout is negligible or may be performed in thebackground), during which time most macroscopic movements of livingorganisms may be considered to be “freezed”. Such movements may includemovements of hearts due to heartbeats, movements of chests due toinhalation and expiration, and movements of legs, arms, backs, and hipsof patients.

[0050] However, the time needed for the recording of two completetwo-dimensional images will not be reduced. Thus any movement during theexamination will affect the images—but both images will be distortedsimilarly (which is important for the dual-energy evaluation).

[0051] If the scanning is performed with a stepwise scanning movement,preferably but not necessarily, two recordings are made at each step ofthe scanning. However, the filter device has only to alter its operationmode once at each step, i.e. at every second line image recording, sincethe recordings at every second step may start with the filter in oneoperation mode and the recordings at the other remaining steps may startwith the filter in the other operation mode.

[0052] For this solution, the misalignment in the pixel-to-pixelcomparison will not generally be present at all, whereas the two-linedetection time will be as indicated above.

[0053] The dual-energy detector apparatus can be used to measure thebone mineral density of humans. Normally the bone is surrounded by anunknown amount of tissue. One commonly used method to estimate theamount of bone is to detect transmitted X-rays at two different X-rayenergies. One image is recorded at X-ray energies where bone and tissueabsorb X-rays very differently, typically at about 40 keV where thedifferential absorption coefficient has a maximum. Another image isrecorded at energies where bone and tissue absorb X-rays in a similarway, typically at energies above 70 keV. From these two measurements theamount of bone is estimated. Since the two separate images are recordedsimultaneously, i.e. each pair of line images at the two differentenergies are recorded close in time, problems due to movements of theobject between the two exposures are heavily reduced. This is oftenachieved by making two separate exposures using two differentacceleration voltages of the X-ray tube and different filters in thebeam path.

[0054] With the current invention the dual-energy measurements are madesimultaneously (or nearly simultaneously) using a single accelerationvoltage of a single X-ray tube (normally 70 kV or higher) and aspecifically designed filter to produce a low energy spectrum and a highenergy spectrum, respectively, simultaneously (or nearlysimultaneously).

[0055] It shall further be appreciated that while the detector unit inthe description above has been described as a gaseous-based ionizationdetector, wherein the freed electrons are drifted in a directionessentially perpendicular to the direction of the incident radiation,the present invention is not limited to such a detector. In fact,virtually any kind of detector can be used in any of the preferredembodiments of the present invention as long as it is a detector capableof recording one-dimensional images of ionizing radiation, to which itis exposed. Examples of such detectors are scintillator-based detectors,PIN-diode arrays, TFT (thin film transistor) arrays, CCD (chargedcoupled device) arrays, CMOS circuits, or any other type ofsemiconductor devices.

[0056]FIG. 3 is a schematic enlarged cross-sectional view similar to theFIG. 2 cross-sectional view, but which illustrates portions of adetector apparatus based on a linear semiconductor array 16′, which maybe used in the dual-energy scanning-based detector apparatus of thepresent invention.

[0057] With reference next to FIGS. 4-6, which are schematic top planviews of a detector arrangement, a collimator arrangement, and a filterarrangement, respectively to be used in an apparatus for dual-energyscanning-based X-ray imaging yet a preferred embodiment of the presentinvention will be described.

[0058] The apparatus for dual-energy scanning-based X-ray imaging may besimilar to the FIG. 1 apparatus, but where the filter device 14, thecollimator 13, and the detector unit 16 are exchanged for thearrangements of FIGS. 4-6.

[0059] The detector arrangement includes a plurality of line detectorunits 16 arranged on a common support structure 42 in a two-dimensionalarray with their respective entrance windows 30 facing upwards. Forillustrative purposes FIG. 4 only includes a matrix of 4×10 detectorunits, i.e. each row 44 includes four detector units and each stack 45includes ten detector units 16, even though it shall be appreciated thatthe arrangement may include many more units. For instance if thedetector units in each stack 45 are spaced apart by s₁=4 mm and an areaof typically 20×20 cm² shall be covered, each stack may include 50detector units. The width of each line detector unit may for instance be40-60 mm.

[0060] It shall be noted that the detector units 16 in each row 44 arepreferably arranged in a staggered manner. If the detector units 16 arenot capable of detecting at their extreme side portions e.g. due to thepresence of sidewalls or spacers, the staggering of the units providesfor complete coverage and any “dead” zones are avoided. Where theentrance slit of one detector unit ends in each row 44, the entranceslit of a further detector unit begins. Nevertheless, the presentinvention is fully applicable to detector arrangements having detectorunits in other stacked configurations.

[0061] It shall be appreciated that instead of arranging multipleindividual detector units 16 with separate gas-tight confinements in thedetector arrangement, a detector arrangement having a common gas-tightenclosing for all individual detector units may be provided (notillustrated). Such a detector box would include the support 42,sidewalls, and a front cover including the entrance windows 30.

[0062] The collimator arrangement, which is optional, is of aradiation-absorbing material 51, e.g. tungsten, and includes a pluralityof radiation transparent slits 52 arranged in rows 53 and stacks 54. Theradiation transparent slits 52 are aligned with the entrance slits ofthe detector units of the FIG. 4 arrangement, so that each planarradiation beam as produced by the collimator 51 when being arranged inthe path of a radiation beam is transmitted through a respective portionof the object to be examined and is entered into a respective one of thedetector units 16 in the FIG. 4 detector arrangement. The collimator 51is then moved together with the detector arrangement during scanningacross the object in a pivoting or translative movement essentially inthe direction of arrows 47 (FIG. 4) and 57 (FIG. 5) to keep thealignment.

[0063] The inventive filter arrangement comprises an array 61 of filtersections 62, 63 carried by a frame support 64. The filter sections areof two different kinds, where every second filter section 62 in thearray has a first filter characteristic and the other filter sections 63in the array have each a second filter characteristic. The filterarrangement is aligned with the collimator arrangement so that each ofthe planar radiation beams will have been filtered by a respective oneof the filter sections when impinging on the object. Note that thefilter arrangement of FIG. 6 does not need a particular moving mechanism(as the filter device of FIG. 1), but is moved only in accordance withthe scanning movement as indicated by arrow 67.

[0064] The alignment between the radiation source (point source, linesource or 2D source), the filter arrangement, the collimatorarrangement, and the detector arrangement provides for collimated androw-selectively filtered multiple planar radiation beams entering theindividual detector units 16 of the detector arrangement. Thus provideddivergent radiation the detector units are arranged to point towards theradiation source used such that radiation from the radiation source canenter the respective detector unit. For the same reason the collimator51 has slits that are less spaced apart than the detector units andnarrower that the detector unit entrance slits, and the width of thefilter sections of the filter arrangement is smaller than the width ofthe rows of the detector arrangement to obtain a proper alignment.

[0065] Scanning is performed at least a distance corresponding to twotimes the distance between two adjacent line detector units 16 in eachstack to record a sufficient number of line images to obtain a completetwo-dimensional image for each one of the two kinds of filtration (astypically one two-dimensional image is obtained when scanning a distancecorresponding to the distance between two adjacent line detectors in astack.

[0066] For such a solution, the misalignment in the pixel-to-pixelcomparison will not generally be present at all, whereas the two-linedetection time will correspond to the time of scanning a distancecorresponding to the inter-detector unit distance s₁, which for thispurpose should be very short. The total scanning time is reduced by afactor corresponding to half the number of the detector units in eachstack.

[0067] Note that in the embodiment described above, a detector arraycomprising a single stack of detector units may be employed instead ofthe FIG. 4 arrangement. Obviously, the collimator arrangement, andoptionally the filter arrangement, has to be modified in such aninstance.

[0068] It shall be appreciated that a further alternative is to employ adetector arrangement comprising only two detector units 16 very closelytogether in a short detector stack as illustrated in FIG. 7. Thecollimator arrangement 71 and filter arrangement 72 are modifiedaccordingly to only comprise two collimator slits 71 a-b and two filtersections 72 a-b, respectively. Scanning has now to be performed adistance equal to the object size in the direction of scanning. However,the distance between the detector units may be made very short withouthaving to bring together a very large number of detector units. Here,the two-line detection time may be very short to the expense of aprolonged total detection time.

[0069] It shall further be appreciated that ideas from the FIGS. 1-2embodiment (or the FIG. 3 embodiment) and the FIGS. 4-6 embodiment maybe brought together to form yet two other preferred embodiments of thepresent invention.

[0070] In the first of these embodiments the detector arrangement andcollimator arrangement of FIGS. 4-5 are used together with a filterarrangement as illustrated in FIG. 8. The filter arrangement of FIG. 8has ten rows 81 (corresponding to the number of rows in the detectorarrangement), each including two filter sections 82, 83 having differentfilter characteristics. The filter arrangement is provided with aparticular moving mechanism (similar to the one described with referenceto FIGS. 1-2) and connected to the scanning movement so that, duringscanning, each of the ionizing radiation beams will have been filteredby a respective filter row, alternately using the two filter sectionsthereof, when impinging on the object. The scanning, which may becontinuous or stepwise, is performed at least a distance correspondingto the distance between two adjacent detector units in the stacks.

[0071] The misalignment, i.e. the pixel-to-pixel correspondence, in thedual-energy image evaluation and the time needed to record two adjacentdual-energy line images are similar to the FIGS. 1-2 embodiment, but thetotal scanning time will be reduced by a factor corresponding to thenumber of detector units in each stack.

[0072] In the second of these embodiments the filter device 14 in theapparatus of FIGS. 1-2 is exchanged for a filter arrangement asillustrated in FIG. 9. This filter arrangement comprises an array 91with a large number of filter sections of two alternating kinds 92, 93having different filter characteristics. The filter arrangement isintended to be fixedly mounted to the vertical stand 18, i.e. kept stillduring scanning. The distance between adjacent filter sections ispreferably adapted to the scanning step ss so that every second lineimage recording is performed with radiation filtered by one kind offilter section, and the other line image recordings are performed withradiation filtered by the other kind of filter section (only onerecording is performed at each scanning step independent of whether thescanning is continuous or stepwise).

[0073] This embodiment is very similar to the continuous scanningtechnique using the FIGS. 1-2 embodiment, but instead of moving thefilter device “on top” of the scanning movement, the filter is heldcompletely still during the scan (and is provided with a structureadapted to the scanning step used).

1. A dual-energy scanning-based radiation detecting apparatuscomprising: a line detector; a device for scanning said line detectoracross an object to be examined while said line detector is exposed toan ionizing radiation beam, which has impinged on said object, tothereby record a plurality of line images of said object; a filterdevice arranged in the path of said ionizing radiation beam upstream ofsaid object to filtrate said ionizing radiation beam, the filter devicebeing capable of operating in two operation modes having differentfilter characteristics; and a control device for altering operation modeof the filter device subsequent to at least every second of said lineimages being recorded.
 2. The apparatus of claim 1 wherein said filterdevice comprises at least two filter sections which sections are capableof being arranged, one at a time, in the path of said radiation beam toform said two operation modes.
 3. The apparatus of claim 2 wherein saidcontrol device comprises a moving mechanism for inserting said at leasttwo filter sections, one at a time, in the path of said radiation beam.4. The apparatus of claim 1 wherein said device for scanning said linedetector across said object is adapted to scan said line detector in acontinuous movement.
 5. The apparatus of claim 4 wherein said linedetector is adapted to be read out at regular intervals to therebyrecord said plurality of line images of said object.
 6. The apparatus ofclaim 4 wherein said control device is adapted to alter the operationmode of said filter device subsequent to each one of said line imagesbeing recorded.
 7. The apparatus of claim 1 wherein said device forscanning said line detector across said object is adapted to scan saidline detector in stepwise movements; and said line detector is adaptedto be exposed and read out when being still between each of saidstepwise movements to thereby record said plurality of line images ofsaid object.
 8. The apparatus of claim 7 wherein said control device isadapted to alter the operation mode of said filter device once whilesaid line detector is still between each of said stepwise movements ateach to each one of said line images being recorded; and said linedetector is adapted, when being still between each of said stepwisemovements, to be exposed and read out once before and once after saidoperation mode alteration performed while said line detector is still.9. The apparatus of claim 1 wherein said line detector is agaseous-based ionization detector, wherein electrons freed as a resultof ionization by said ionizing radiation beam are accelerated in adirection essentially perpendicular to the direction of said ionizingradiation beam.
 10. The apparatus of claim 1 wherein said line detectoris any of a scintillator-based detector, a PIN-diode array, a TFT array,a CCD array, a liquid-based detector, and a solid-state detector. 11.The apparatus of claim 1 comprising an X-ray tube for producing saidionizing radiation beam, said X-ray tube having an operating voltage,which is kept essentially constant during the scanning of said linedetector across the object to be examined.
 12. A dual-energyscanning-based radiation detecting apparatus comprising: a plurality ofstacked line detectors; a device for scanning said plurality of stackedline detectors across an object to be examined while each of saidplurality of stacked line detectors is exposed to an ionizing radiationbeam, which has impinged on said object, to thereby record a pluralityof line images of said object; a filter device arranged in the path ofsaid ionizing radiation beams upstream of said object to filtrate saidionizing radiation beams, the filter device comprising an array offilter sections aligned with said ionizing radiation beams duringscanning so that each of said ionizing radiation beams will have beenfiltered by a respective one of said filter sections when impinging onsaid object, where every second filter section in the array has a firstfilter characteristic and the other filter sections in the array haveeach a second filter characteristic; and said device for scanning saidplurality of stacked line detector is arranged to scan at least adistance corresponding to two times the distance between two adjacentline detectors of said plurality of stacked line detectors.
 13. Theapparatus of claim 12 wherein said plurality of stacked line detectorsis two; said two stacked line detectors are arranged closely together;said filter device comprises two filter sections aligned with saidionizing radiation beams during scanning so that each of said ionizingradiation beams will have been filtered by a respective one of said twofilter sections when impinging on said object, where said two filtersections have different filter characteristics; and said device forscanning said two stacked line detector is arranged to scan across theentire length of said object in the direction of the scanning.
 14. Theapparatus of claim 13 wherein said two stacked line detectors arearranged up against each other.
 15. The apparatus of claim 12 comprisingan X-ray tube for producing said ionizing radiation beam, said X-raytube having an operating voltage, which is kept essentially constantduring the scanning of said plurality of stacked line detectors acrossthe object to be examined.
 16. A dual-energy scanning-based radiationdetecting method comprising the steps of: scanning a line detectoracross an object while said line detector is exposed to an ionizingradiation beam, which has impinged on an object to be examined, tothereby record a plurality of line images of said object; arranging afilter device in the path of said ionizing radiation beam upstream ofsaid object to filtrate said ionizing radiation beam, the filter devicebeing capable of operating in two operation modes having differentfilter characteristics; and altering operation mode of said filterdevice subsequent to at least every second of said line images beingrecorded.
 17. A dual-energy scanning-based radiation detecting methodcomprising the steps of: scanning a plurality of stacked line detectorsacross an object to be examined while each of said plurality of stackedline detectors is exposed to an ionizing radiation beam, which hasimpinged on said object, to thereby record a plurality of line images ofsaid object; arranging a filter device in the path of said ionizingradiation beams upstream of said object to filtrate said ionizingradiation beams, the filter device comprising an array of filtersections; and having said filter sections aligned with said ionizingradiation beams during scanning so that each of said ionizing radiationbeams will have been filtered by a respective one of said filtersections when impinging on said object, where every second filtersection in the array has a first filter characteristic and the otherfilter sections in the array have each a second filter characteristic;wherein said scanning said plurality of stacked line detector isperformed at least a distance corresponding to two times the distancebetween two adjacent line detectors of said plurality of stacked linedetectors.